Gas turbine engines are generally known in the art and used in a wide range of applications, such as propulsion engines and auxiliary power unit engines for aircraft. In a typical configuration, a turbine section of the gas turbine engine includes a turbine nozzle, etc. A turbine nozzle comprises an annular array of stationary airfoils (also referred to herein as “vanes”) that extend between annular endwalls. In the gas turbine engine, hot combustion gases from a combustion section in the gas turbine engine are directed against the annular array of vanes. When the vanes are heated faster or hotter than the endwalls, the vanes become susceptible to large thermal compressive stresses because the vanes tend to expand but are constrained by the endwalls. Therefore, conventional bi-cast turbine nozzles include a slip joint and associated space between an end portion of each vane in the annular array and the adjacent endwall to accommodate thermal expansion of the vanes. The opposing end portion of each vane is mechanically anchored into an opposing endwall. The slip joint, when in an open condition, forms a gap along a pressure sidewall of the vane and an opposing gap along a suction sidewall of the vane.
While the slip joint between the end portion of each of the vanes and the adjacent endwall in the bi-cast turbine nozzle is generally provided to accommodate thermal expansion of the vanes, the slip joints can undesirably allow for hot combustion gas ingestion from the pressure side of the vanes, into the associated space, and onto the suction side of the vanes. Such hot combustion gas ingestion can result in aerodynamic performance degradation and oxidation damage to the vanes and adjacent endwall at the slip joints, causing material recession of the vanes and adjacent endwall. As the size of the gaps between the end portion of the vanes and adjacent endwall at the slip joint increases due to material recession, the amount of hot gas ingestion increases, resulting in still higher aerodynamic performance degradation and even more oxidation damage, continuing to cause even more recession.
Film cooling of vanes is a widely used technique that helps to maintain material temperatures within acceptable limits. With film cooling of vanes, air is extracted from a compressor section of the gas turbine engine and forced through internal cooling passages within the vanes before being ejected through a showerhead or other film cooling holes in the vane onto the external wall surfaces of the vane. The cooling gas ejected from these film cooling holes forms a film layer of cooling gas on the external wall surfaces to protect the vane from the hot combustion gases by substantially reducing heat transfer from the hot combustion gases to the vane skin as the cooling gas is at a lower temperature than the hot combustion gas. Film cooling of endwalls using endwall film cooling holes is also known. Cooling film blow-off (i.e., separation of the cooling film layer from the vane and/or endwall external wall surfaces) may, however, substantially impede formation of the film layer of cooling gas against the external wall surfaces, resulting in lower overall vane/endwall cooling effectiveness. In addition, neither vane film cooling nor endwall film cooling sufficiently cool the slip joint of the bi-cast turbine nozzle to avoid the aerodynamic performance degradation and oxidation damage that are caused by the hot gas ingestion through the slip joints and through the associated space in the endwall.
Hence, there is a need to substantially prevent oxidation damage caused by hot gas ingestion at the slip joints of bi-cast turbine nozzles, to thereby maintain aerodynamic performance and operative life of the bi-cast turbine nozzle. It is also needed to mitigate cooling film blow-off, thereby resulting in higher overall cooling effectiveness. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the present invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.